The eye is lined with light-sensitive cells. These are found at the back of the eye on the retina. Over these cells are the blood vessels and nerves that serve the eye. You can see these blood vessels in the following experiment.
Give every student a small square of aluminium foil. This can be placed in a frame to stop it tearing.
You can make a frame with two square pieces of card, 5cm by 5cm. Place these together and fold in half. Cutting from the folded edge, remove a 3.5cm by 3.5cm piece from the card. Unfold the card and sandwich the aluminium foil between the two pieces and glue in place. Make a small pinhole in the aluminium foil. You may need to experiment a bit with the size of the hole - using a sharp pencil tip makes a hole of about the right size.
Use an OHP to set up a brightly lit screen (a well-lit whiteboard works equally well). Ask students to close one eye, and then place the pinhole in front of their open eye. Hold the pinhole quite close to the eye and look at the bright screen. By carefully moving the pinhole you should see lots of blood vessels that will look like the veins in a leaf.
Amazingly, we don't notice the blood vessels blocking light from reaching the retina. Our brains are more sensitive to changes in the light entering the eye, perhaps because this is more important when looking for food or avoiding predators.
You can find your blind spot really easily. Write a list of numbers on a piece of paper from the middle to the outer edge. Draw a cross half way between the lefthand side of the paper and the row of numbers. Close your left eye and look at the cross with your right eye. Carefully move the paper towards you. You will find that one of the numbers disappears when it hits your blind spot.